The invention refers to an apparatus for the three-dimensional determination of flows using a light source for briefly forming a sheet of light or light section and having a stereoscopic recording device which has two equivalent objectives with optical axes aligned parallel to each other as well as perpendicular to the plane of the light section, and located to receive light from the objectives, two optical sensor fields aligned perpendicular to the plane. The apparatus also includes a forward displacement or image shifting device for the synchronized forward displacement of the projections of the light source onto the two sensor fields. In this context the terms parallel and perpendicular are always to be understood as being relative to the optical axes. This means that with an arrangement of intermediate refractive boundary surfaces, a non-parallel or non-perpendicular geometrical arrangement can certainly occur.
The measuring principle behind the aforementioned apparatus is known as the pulsed light method or Particle Image Velocimetry (PIV). In this, the flow to be investigated is first doted with light-scattering particles. In some cases the flow's natural loading of various non-specific particles is sufficient. Then, at the point to be investigated in the flow, a flat space, a so-called light section light sheet, is created twice using the light source. By doing this, optical sensor fields are simultaneously exposed twice and an images of the light section are formed on the optical sensor fields. An objective with an optical axis aligned perpendicular to the plane of the light section is provided for the image, so that the plane of the light section falls in the object plane of said objective and the sensor field is arranged in the focal plane of said objective. The sensor field, exposed twice, now permits the determination of the velocity of the particles carried along by the flow in the plane of the light section. The velocity is always calculated from the separation between two images of a particle on the sensor field and the temporal separation of the two light pulses emitted by the light source, taking into account the imaging characteristics of the objective.
To evaluate the double exposure images of the light section, a two-fold Fourier transform is performed digitally as a fast Fourier transform and/or advantageously quickly as an optical Fourier transform for particular areas of the sensor field, respectively. Here, however, the mathematical sign of the flow velocity of the particles observed is lost in the light section. This is also true for all other cases in which, when the light section is imaged on the sensor field, it cannot be distinguished whether an individual image of a certain particle now in fact belongs to the first or the second illumination by the light source. A further disadvantage of the twin-flash method in its simplest form described here is that the velocities in the three-dimensional flow can only be recorded two-dimensionally, limited to the plane of the light section. To overcome these disadvantages, two modifications of the twin-flash method are known, and these may also be combined.
In order to record the direction of the velocities of the particles transported by the flow, the second image of the light section is forwardly displaced artificially on the sensor field by a certain dimension which at least corresponds to the maximum displacement of the particle opposite to the flow or opposite to the direction of the forward displacement respectively between the two illuminations of the light section. This means that all particle displacements which can be registered on the sensor field have the same mathematical sign and the actual direction of the flow can be determined without any trouble taking into account the size and direction of the forward displacement. Forward displaced twin-flash recordings of the light section can also be particularly easily evaluated by means of two-fold Fourier transform.
In order to make it possible to record in a three-dimensional manner the velocities in the three-dimensional flow, the light section is imaged simultaneously on two separate sensor fields using a stereoscopic arrangement. 0f course, for this two separate objectives must be provided. A special feature of the stereoscopic arrangement results from the fact that the plane of the light section must coincide with the planes of both objectives so that two-dimensional, sharp images of the light section are at all possible on the two sensor fields. Accordingly, the objectives have optical axes that are arranged perpendicular to the plane of the light section and parallel to each other, so that the area of the light section located between the optical axes of the objectives is imaged stereoscopically on the sensors fields arranged outside the optical axes and aligned perpendicular to the plane of the light section. Therefore, although the particles transported by the flow are only illuminated in a comparatively flat space, a determination of the flow in all three dimensions is possible. For this, the two twin-flash recordings of the light section on the two sensor fields are evaluated parallel to each other and under mutual consideration. It is obvious that there is a limit to the three-dimensional resolution if the particles transported by the flow leave the flat, narrow space of the light section laterally between the two illuminations of the light section. This always happens if the velocity of the particles perpendicular to the plane of the light section is greater than the quotient arising from the thickness of the light section and the temporal separation of the two flashes of light emitted by the light source for illuminating the light section. However, with simultaneous forward displacement of the second image of the particles, the temporal separation of the flashes of light can be made much shorter without falling below the minimum separation between the two images which must be maintained for the evaluation. Therefore, the number of particles leaving the light section between the two flashes of light is reduced and, at the same time, the signal-to-noise ratio upon evaluation is improved by two-fold Fourier transform.
An apparatus of the aforementioned type, which is suitable for performing a stereoscopic twin-flash method with forward displacement, is known from the article "Stereoscopic Particle Image Velocimetry Applied to Liquid Flows" (Prassad, A. K.; Adrian, R. J., 6th International Symposium on Applications of Laser Technique to Fluid Mechanics, Lisbon, Portugal, 20 to 30 Jul. 1992, paper 6-1). This apparatus serves for determining a flow in a fluid, whereby the objectives and the sensor fields are arranged outside the fluid. Accordingly, the light scattered by the particles in the flow is refracted at the boundary surface of the fluid. This is compensated for by angling the sensor fields relative to a plane parallel to the plane of the light section. On the other hand, the optical axes of the two objectives are aligned both optically and geometrically perpendicular to the plane of the light section and parallel to each other. The forward displacement device in the known apparatus only displaces the sensor fields. This is costly, particularly with regard to the synchronization, inasmuch as the sensor fields are not arranged in a common plane. Furthermore, the maximum velocity with forward displacement is severely limited, meaning that only the observation of comparatively slow flows is possible. Although no partcularly high flow rates occur in three-dimensional flows in fluids, totally different relationships result from, for example, flows in wind tunnels.
An apparatus for determining two-dimensional flows using a light source for briefly illuminating a light section and having a recording device which has an objective aligned perpendicular to the plane of the light section and an optical sensor field aligned parallel to the plane behind the objective, whereby a forward displacement device having a revolving mirror is provided for the image of the light section on the sensor field, is known from the article "Measuring Turbulence in Reversing Flows by Particle Image Velocimeter" (Gran, I. et al., ICALEO 92/L.I.A. vol. 68, 1989). The revolving mirror is arranged in the path of the beam between the light section and the sensor field. With the double exposure of the light section the revolving mirror rotates so that a beam of light emanating from the same point of the light section is projected onto two different places on the sensor field. This corresponds to a displacement of the image of the light section on the sensor field. However, according to the article mentioned above, a forward displacement device with a revolving mirror can only be employed for flow rates up to 30 m/s. In wind tunnels values of more than 100 m/s are reached. Moreover, the known apparatus, as was already mentioned, is only suitable for the determination of two-dimensional flows.
The restriction on a forward displacement device with a revolving mirror to applications in which the flow rates do not exceed 30 m/s, results from the article (Image Shifting Technique to Resolve Directional Ambiguity in Double-pulsed Velocimetry" (Adrian, R. J., Applied Optics, Vol. 25, No. 21, 1 Nov. 1986). This article is concerned with an apparatus for the determination of two-dimensional flows which, in terms of all the essential points, correspond to those already described.